This invention relates to the improved protection of a secondary distribution system by automatic coordination of the static trip circuit breaker characteristics. When an "upstream" breaker is feeding only "downstream" breakers, such that no loads per se, are connected directly to the upstream breaker, the upstream breaker can also provide low-current protection for phase-to-phase or phase-to-neutral faults occuring between the upstream and downstream breakers, in addition to and analagous to ground fault protection.
To sense a ground fault condition, one known approach is to compare the total current flowing in the distribution circuit from the source to the load with the total current returning from the load to the source. A ground fault exists if an imbalance or inequality in these currents is found, since some of the current leaving the source is then flowing through the ground fault and returning to the source via an extraneous ground circuit path and the ground return conductor connecting the source to ground. One way of detecting this current imbalance is to link all the distribution circuit conductors with a "zero sequence" differential current transformer core and look for a current induced in a secondary winding thereof by residual flux flowing in the core. An alternative approach to detecting this current imbalance is to connect the secondary windings of individual phase current transformers and a neutral current transformer, if a neutral conductor is present, into a summing circuit. If the vectorial summation of the secondary currents is zero, no ground fault conditions exist. However, if the vectorial summation of these secondary currents is not zero, then a ground fault condition is indicated. The differential secondary current is then sensed to develop a ground fault signal for processing pursuant to initiating a ground fault trip function. A good description of circuits employed for both overcurrent and ground fault protection is given in U.S. Pat. No. 4,271,444 entitled "Ground Fault Trip Mode Network For Static Trip Circuit Breakers" in the name of E. K. Howell, which patent is incorporated herein for purposes of reference.
The concept of zone interlock coordination between static trip circuit breakers, wherein tripping of an upstream breaker is delayed in order to give the downstream breaker closest to the fault occurrence sufficient time to trip, is also described in the aforementioned patent to Howell, as well as within later issued U.S. Pat. No. 4,468,714 entitled "Zone Selective Interlock Module For Use With Static Trip Circuit Breakers" in the name of Ronald R. Russell. This patent is incorporated herein for reference purpose and should be reviewed for a good description of the wiring interconnection between a plurality of upstream and downstream breakers for coordinating the short time and ground fault zone selective interlock functions of the breakers by selective adjustment of the trip time response characteristics of the upstream and downstream breakers.
One purpose of the instant invention is to describe a simplified circuit and wiring interconnection between a plurality of downstream circuit breakers and an upstream breaker for providing low current protection for phase-to-phase or phase-to-neutral faults occuring between the upstream and downstream breakers.